1. Field of the Invention
The present invention relates to the measurement and testing of structures for adequacy of lightning protection. In particular, the present invention provides an apparatus and method for accurately determining theoretical energy levels that a lightning strike causes within a tested structure and the need for appropriate remedial measures to protect personnel and critical assets.
2. Description of Related Art
In general, there is no systematic, reliable and accurate quantitative method available to determine the degree of lightning protection afforded by a structure, an external lightning protection system (LPS) for that structure, or for the combination of the structure and the lightning protection system.
Mardiguian, Grounding and Bonding, pps. 10.27 to 10.29, and Plumey, et al., xe2x80x9cHigh Frequency Harmonic Input Impedance Of An Antenna Embedded In A Conducting Half-Spacexe2x80x9d, Electromagnetic Compatibility, 1983 Proceedings, pp. 45-50, both discuss grounding lightning transients and refer to measuring ground impedance at frequencies above dc. The measurement is made using a grounding rod under test, an auxiliary grounding rod providing a current return, a reference grounding rod, a surge generator for injecting a current impulse through a matched resistor between the grounding rod under test and the first auxiliary grounding rod, and an oscilloscope or spectrum analyzer for measuring the voltage transients across a voltage divider network connecting the ground rod to be tested and the second auxiliary grounding rod. The impedance is determined from the ratio v/I of the probe to earth voltage to the injected current transient.
As a modeling methodology, this method suffers from the problem that how lightning strikes affect structures was not well understood until recently. Recent testing has demonstrated that a substantial amount of the energy of a lightning strike hitting a structure is dissipated in the metallic support elements of the structure, rather than the external lightning protection system.
Because the flow of lightning energy through a structure can damage or destroy critical assets such as sensitive electronic equipment or explosive materials within the structure, additional analysis has proven necessary.
Indeed, the most commonly used prior art lightning protection system measurement and testing method requires the measurement of the continuity and ground resistance from an air terminal to surrounding earth using the fall-of-potential method with an alternating current ohmmeter. The resistance value must be less than 10 to 25 Ohms, depending upon the governing regulation. A rigorous visual and mechanical inspection of all of the air terminals, conductors, fasteners and ground rods is necessary to assure the mechanical robustness of the external lightning protection system. No quantitative protection level can be calculated from such a method. Other versions of this ohmic technique require disassembling various individual components and conductors and measuring the continuity of key elements in the external lightning protection system, as well as the resistance to ground.
Prior U.S. Patents that deal with testing of a lightning protection system generally involve either direct current injection of a test signal or magnetically inducing a current in elements of the lightning protection system. For example, U.S. Pat. No. 4,142,143 teaches a method of measuring the admittance (conductance and reluctance) of a conductor to determine its continuity within a larger lightning protection system without the disassembly of the structure. Another example is U.S. Pat. No. 5,654,641, which teaches a method that measures the current division within a lightning protection system to qualitatively estimate whether protection is affordable in the vicinity of the particular conductor. U.S. Pat. No. 5,929,625 teaches a method of monitoring a lightning protection system with a device that continually monitors the lightning protection system using a magnetic circuit.
None of these methods provides accurate, quantitative measurements that can be used to calculate the expected energy levels within the structure from a lightning strike or recommended modification of either locating critical materials within the structure or ways for modifying a structure or the combination of the lightning protection system and the structure to prevent damage when a lightning strike occurs.
Other prior art U.S. Patents that deal with automated testing of grounding systems at frequencies above dc using automated control apparatus include U.S. Pat. No. 5,365,179. This reference teaches a method and apparatus for measuring ground to earth impedance at frequencies above dc using a computer controlled instrument to implement a three point measurement or a fall-of-potential method using three ground paths and a plurality of discrete frequencies in the range of from 5 Hz to 200 MHz. The complex impedance vector array for each frequency selected in the range may be determined for identifying antenna effects to improve the quality of the ground and improved ground path selection for an electrical system. This patent does not describe methods of determining the lightning protection effectiveness of a structure and its lightning protection system, nor does it recognize methods for deriving a transfer function for a structure, the characteristics of a lightning strike and how it interacts with that structure or problems resolved by the instant invention.
One of the problems these prior methods represent is that they do not model accurately a structure with metallic elements or take into account that these metallic structural elements dissipate most of the energy incurred by a lightning strike. The present invention provides a direct, accurate and quantitative measurement and determination of protection of a structure by deriving a collection of transfer functions at various locations within the structure and using modeled excitation functions in the frequency spectrum of lightning that are convolved with these transfer functions to predict the affects of a worst case lightning strike. This information provided useful standoff distances for minimizing harm to personnel and damage to critical assets located within that structure.
The present invention provides an apparatus and measurement methodology that allows accurate, safe and practical determination of the degree of lightning protection for a structure having metallic structural elements. This is particularly useful for a structure that requires zones of safety for personnel or critical materials, such as computers, explosives or other critical assets. The apparatus in cooperation with frequency domain analytical method yields a quantitative protection level for a structure. The method of the invention includes:
i) determining the test locations, such as an air-terminal (lightning rod) or other metallic conductors at the top of the structure, for the measurement of the transfer functions of the structure;
ii) injecting a low-level test current into the test locations at multiple test frequencies and allowing the injected test current at each test frequency to flow through the metallic and other partially-conductive elements of the structure, and flow to the surrounding earth beneath the structure;
iii) measuring the electromagnetic fields (electric field, magnetic field, or both); The method of the invention includes:
i) determining the test locations, such as an air-terminal (lightning rod) or other metallic conductors at the top of the structure, for the measurement of the transfer
iv) calculating and synthesizing the transfer functions for each test location within the structure; and,
v) determining the internal energy levels that lightning strikes would cause inside the structure using probable models of a lightning event at these various test points.
The collection of transfer functions from the various injection points of measurement locations can then be derived using the method of the invention, from which the effect of an extreme lightning strike can be calculated. The safe separation distance from walls, ceilings, or other conductive surfaces can be calculated from the calculated energy levels expected in a lightning strike event.
The resulting a priori energy levels a lightning event presents to the structure provides information to modify the lightning protection system of the structure. Alternatively, the critical locations where people reside may be relocated within the structure, or materials or equipment located within the structure may be relocated, providing enhanced safety for the personnel and minimizing damage to critical assets located within the structure.
The apparatus of the present invention includes a signal generator to inject test currents at multiple frequencies into the structure and a calibrated antenna and frequency spectrum analyzer to measure the electromagnetic fields created by the excitation signal for several frequencies within the spectral region of lightning. This is done using either manual or automated computer control of the signal generator in association with data compilation at each particular frequency for determining the transfer function at that location.
It is an object of the present invention to provide a frequency-domain measurement testing apparatus using either manual or automated embodiments of the apparatus to determine the equivalent collection of transfer functions of multiple test points within a structure (such as an airport hanger, auditorium, warehouse, ammunition bunker or other structure having metallic structural elements) in the frequency spectral region occupied by lightning strikes to provide information of such strikes for modifications of the structure""s lightning protection system or location of critical materials located therein.
It is a further object of the present invention to provide a testing method that includes exciting a structure with metallic structural elements using low-amplitude signals, within the same frequency range as lightning strike currents, to make effective measurements for deriving an electronic transfer function for determining how a lightning event will affect a structure, thus providing a non-destructive test method for modifying the structure to enhance safety and minimize damage to critical assets.
It is a still further object of the present invention to provide a testing method that includes using a continuous-wave signal generator to inject excitation current into any exterior metal conductor on the structure for deriving a lightning protection system transfer function.
It is another object of the present invention to provide a testing method that includes using an electric-field antenna within the structure to measure the resulting electric field generated by an excitation current.
It is another object of the present invention to provide a testing method that includes using a fiber optic data link to eliminate spurious pick-up by long cables, acting as parasitic antennas for deriving a lightning protection system transfer function for accurate determination.
It is another object of the present invention to provide a testing method that includes using a frequency spectrum analyzer to detect the low energy levels from an antenna and to reject noise and other spurious signals from corrupting the measurement for deriving a lightning protection system transfer function for accurate determination.
It is another object of the present invention to provide a testing method that includes using a magnetic-field antenna within the building to measure the resulting magnetic field generated by an excitation current.
It is another object of the present invention to provide a testing method that includes using a calibrated radio-frequency receiver in place of the frequency spectrum analyzer to detect the low energy levels from an antenna and to reject noise and other spurious signals from corrupting the measurement for greater accuracy in determining a lightning protection system transfer function.
It is another object of the present invention to provide a testing method that includes using a signal generator coupled to an externally-located magnetic field antenna so as to magnetically excite the structure under test, rather than inject excitation currents through direct connection to the structure as alternative embodiment of the invention.
The other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of the preferred embodiments thereof.
According to a preferred embodiment of the present invention, there is provided an apparatus for the determination of the degree of lightning protection of a structure having metallic structural elements by determining an electromagnetic field transfer function for each test location selected from among metallic conductors, such as lightning rods, located in the proximity of the top of said structure, which apparatus comprises:
a) a signal generation instrumentation subsystem which can be operatively connected to each said test location in succession and the earth surrounding said structure, which signal generation instrumentation subsystem is capable of injecting a low-level test current at a plurality of specific test frequencies into a selected test location;
b) a receiver instrumentation subsystem comprising:
i) an internal component intended to be disposed within said structure to measure the electric field created by the injection of said low-level test current into said selected test location and comprising:
A) an electric field antenna which may be disposed within said structure to measure the electromagnetic field at the position where said antenna is disposed;
B) a fiber optic transmitter operatively connected to said antenna and capable of converting the electromagnetic field measurement of said antenna into an optical signal; and,
C) an optical fiber cable operatively connected to said fiber optic transmitter and capable of conducting said optical signal;
ii) an external component intended to be disposed outside said structure to analyze the electric field created by the injection of said low-level test current into said test location and comprising:
A) an fiber optic receiver operatively connected to said optical fiber cable and capable of converting said optical signal to an electrical signal;
B) a frequency spectrum analyzer operatively connected to said fiber optic receiver and capable of converting said electrical signal to an electric field value in volts RMS (root-mean-square) per meter; and,
c) a computing device operatively connected to said signal generation instrumentation subsystem and said receiver instrumentation subsystem and capable of matching the data received from said receiver instrumentation subsystem with the low-level test current injected at each test location by said signal generation instrumentation subsystem, and analyzing such data to provide an overall assessment of the ability of said structure to adequately dissipate the energy of a lightning strike in order to determine a safe stand-off distance for critical assets or personnel within the structure.
According to another embodiment of the present invention, there is provided a method for the determination of clearances for safety of personnel and safe storage of critical assets within a structure, which method comprises the steps of:
i) evaluating the appropriate test locations for a structure and selecting a plurality of said test locations from among metallic conductors, such as lightning rods, located in the proximity of the top of said structure;
ii) equipping said structure with a test apparatus comprising:
a) a signal generation instrumentation subsystem which can be operatively connected to each said test location in succession and the earth surrounding said structure, which signal generation instrumentation subsystem is capable of injecting a low-level test current at a plurality of specific test frequencies into a selected test location;
b) a receiver instrumentation subsystem comprising:
i) an internal component intended to be disposed within said structure to measure the electric field created by the injection of said low-level test current into said selected test location and comprising:
A) an electric field antenna which may be disposed within said structure to measure the electromagnetic field at the position where said antenna is disposed;
B) a fiber optic transmitter operatively connected to said antenna and capable of converting the electromagnetic field measurement of said antenna into an optical signal; and,
C) an optical fiber cable operatively connected to said fiber optic transmitter and capable of conducting said optical signal;
ii) an external component intended to be disposed outside said structure to analyze the electric field created by the injection of said low-level test current into said test location and comprising:
A) an fiber optic receiver operatively connected to said optical fiber cable and capable of converting said optical signal to an electrical signal;
B) a frequency spectrum analyzer operatively connected to said fiber optic receiver and capable of converting said electrical signal to an electric field value in volts RMS (root-mean-square) per meter; and,
c) a computing device operatively connected to said signal generation instrumentation subsystem and said receiver instrumentation subsystem and capable of matching the data received from said receiver instrumentation subsystem with the low-level test current injected at each test location by said signal generation instrumentation subsystem, and analyzing such data to provide an overall assessment of the ability of said structure to adequately dissipate the energy of a lightning strike in order to determine a safe stand-off distance for critical assets or personnel within the structure;
ii) injecting a plurality of low-level test currents into each said test locations, each said test current at a specific test frequency and allowing each said injected test current at each specific test frequency to flow through any metallic or other conductive elements of said structure, into the surrounding earth beneath the structure;
iii) measuring the electromagnetic fields;
iv) calculating the transfer functions for each test location selected for said structure; and,
v) determining the internal energy levels that lightning strikes would cause inside the structure using probable models of a lightning event at these various test points in order to determine a safe stand-off distance for critical assets or personnel within said structure.